Vertical reciprocating conveyors are employed
warehouses, factories, and the like to convey materials between different vertical levels. The typical vertical conveyor includes a supporting structure or frame and a carriage, which is adapted to support a cargo or load, is guided for vertical movement on the supporting structure. The carriage can be moved upwardly and downwardly on the structure by either a mechanical or hydraulic drive. In one common form of vertical conveyor, the carriage or platform is cantilevered from the supporting structure, and lifting and lowering is accomplished through a single cylinder attached through a pair of roller chains to the carriage. The chains are arranged, such that there is a 2:1 relationship between carriage movement and piston rod extension.
The cantilevered type of vertical conveyor has a serious drawback, in that it cannot accommodate heavier loads. As all of the weight of the load is cantilevered from the supporting structure, the forces on the carriage and the guide mechanism are extremely high resulting in possible distortion of the guide structure and the carriage, and unacceptable wear on the guide rollers, bearings, etc.
In a second form of vertical conveyor, the load carrying platform or carriage is straddled between two vertical supports or beams. This results in a balanced load, so that the stress on the supporting structure, guide rollers and bearings is minimal.
With the typical straddle design, lifting and lowering of the carriage is accomplished through the use of two hydraulic cylinders, each mounted on one of the vertical beams. The piston rod of each cylinder unit is connected through a sheave and wire cable arrangement to the carriage, in a manner such that the carriage is elevated by retraction of the piston rods. The "pull mode" of lifting the carriage is more stable than a "push mode", for in high rise applications using a "push mode", the ram or piston rod can be extended to a range of about ten feet, with the result that there is considerable load on the packing glands of the cylinders, as well as a high bending force that is imparted to the extended piston rod or ram.
With the straddle-type of vertical conveyor using a pair of hydraulic cylinder units, there is a problem in synchronizing the action of the two cylinder units. While there are hydraulic devices to accomplish synchronization, these devices require sophisticated valving and electronic programming and are not appropriate in terms of cost for a vertical conveyor.
The absence of cylinder synchronization is most pronounced when there is an eccentric or off-center load on a relatively large carriage. While both cylinders see the same hydraulic pressure, the uneven load makes the cylinders pull unevenly, and as a result, a racking force is introduced to the carriage. The racking force is resisted by guide rollers on the carriage running against a guide track on the upright vertical beams. However, the upright vertical beams, as used in a typical vertical conveyor, are not precisely machined components and are subject to dimensional tolerances of plus or minus 0.125 inch, and as a result cannot serve as precise guides. Further, the vertical beams are often unsupported for as much as twenty feet, and thus an eccentric load on the carriage imposes stresses on the beams and causes severe wear of the guide members that ride on the guide tracks on the beams.
Because of the sheaving arrangement, the cylinders of the typical vertical conveyor are spaced laterally of the vertical beams, and this lateral spacing imparts a bending force to the beams. Because of this, it has been the practice to reinforce the vertical beams using diagonal braces, which increases the manufacturing and installation cost of the conveyor.